1. Field of the Invention
The present invention relates to an image forming apparatus having a sheet re-supplying path for forming at least two images on the same sheet.
2. Related Background Art
Recently, image forming apparatuses in which images are formed on both surfaces of a single sheet have been requested more and more. Among these apparatuses, there is, for example, a laser beam printer (LBP) 1d as shown in FIG. 1.
In FIG. 1, the laser beam printer 1d includes a body 2 having a bottom on which a cassette 3 accommodating sheets S is disposed. A sheet supply roller 5 is arranged above the front part of the cassette 3 to separate and supply the sheets one by one. At a downstream side of the supply roller 5, there are arranged, in order, a pair of regist rollers 6 for maintaining a leading end of the sheet picked-up by the supply roller 5 in a correct position to ensure good recording accuracy of an image, and a process unit 10 comprising functional portions such as a photosensitive drum 7, a developing device 9 and the like.
A laser scanning unit 11 illuminates a laser beam on the photosensitive drum 7 on the basis of image information from an apparatus body (not shown) to form a latent image on the drum. The latent image is developed by the developing device 9 to form a toner image which is in turn transferred onto the sheet S fed by the supply roller 5 and pressed against the photosensitive drum 7 by a transfer roller 12. The sheet S having the transferred image thereon is fixed by a fixing device 13 including fixing rollers arranged at a downstream side of the transfer roller 12, and then is fed to a pair of feed rollers 15 and then to a downstream flapper 16.
If the image is to be printed or recorded on a single surface of the sheet S, as shown in FIG. 1, the flapper 16 is in a position rotated in an anticlockwise direction, where the sheet on which the image has been recorded is directed toward an sheet ejecting path 19 through which the sheet S is ejected onto an ejector tray 17 with a recorded surface thereof turned inside.
On the other hand, if the images are to be recorded on both surfaces of the sheet S, the flapper 16 is rotated in a clockwise direction from the position of FIG. 1 to a "both-surface" position, the sheet S is directed toward sheet guides 22 of a switch-back unit 21 and then is pinched or nipped and fed by reversible rollers 23. When a trailing end of the sheet S is detected by a sensor 25 arranged at a downstream side of the sheet guides 22, the reversible rollers 23 are rotated reversely to feed the sheet in the reverse direction. The sheet S fed in the reverse direction is directed toward a sheet feeding path 26 for a second surface recording (sheet re-supplying path) by a flapper 25 disposed at an upstream side of the reversible rollers 23, and then is fed toward re-supply rollers 29 by means of feed rollers 27 disposed in the sheet re-supplying path.
Incidentally, the flapper is spring-biased to normally occupy the position shown by a solid line in FIG. 1, and the sheet fed from the sheet guides 22 reaches the reversible rollers 23 while urging the flapper 25 downwardly. When the trailing end of the sheet has passed through the flapper 25, the latter is returned to the position shown in FIG. 1.
The sheet feeding path 26 for the second surface recording is driven independently from that of a sheet feeding path 20 for a first surface recording, since it must be controlled independently from the sheet feeding path 20. At a downstream side of the re-supply rollers 29, a sensor 30 for the second surface recording is arranged. When the sensor 30 detects the leading end of the sheet S, the sheet is stopped, whereby the timing of the sheet supply for the second surface recording is controlled.
The sheet re-supplying path 26 includes a path section 26a positioned between the reversible rollers 23 and the feed rollers 27, a path section 26b positioned between the feed rollers 27 and the re-supply rollers 29, and a path section 26c positioned between the re-supply rollers 29 and the regist rollers 6. Since the sheet S is switched back in the switchback unit 21, in the sheet feeding path 26 for the second surface recording, the first recorded surface of the sheet S faces toward the photosensitive drum 7; but, the sheet S is U-turned in the path section 26c so that the second non-recorded surface of the sheet faces the photosensitive drum 7 when the image is recorded on the sheet again.
However, in the above-mentioned conventional arrangement, there arose the following problems:
That is to say, since the image forming apparatus having the sheet re-supplying path has recently been small-sized, the sheet feeding path for the second surface recording is normally shorter than the twice of the maximum length of the sheet S to be re-supplied. As a result, when the first sheet S is waiting for the second surface recording in the sheet re-supplying path 26, the second sheet S cannot be fed to the sheet feeding path 20.
Accordingly, when the images are recorded on both surfaces of the same sheet, the recording is effected, in order, for the first surface of the first sheet, second surface of the first sheet, first surface of second sheet, second surface of the second sheet, . . . first surface of (N-1)th sheet, second surface of the (N-1)th sheet, first surface of Nth sheet, second surface of the Nth sheet, and so on. In this way, since the first surface of a new sheet cannot be recorded till after the second surface of the previous sheet has been recorded, the through-put was considerably worsened when the image were on both surfaces of the same sheet.